Scanning the area constantly will let you see targets that you might have missed by just staring forward.
Reload in "quiet" periods as several guns don't have enough bullets to complete a challenge without reloading When lw on bullets, waste them all during a "quiet" part so that you can reload with full ammo by the time the targets continue.
Guns are quite inaccurate, while some guns list their stats and say their accuracy is awful. It usually is worse than other guns, but not awful at all.
Up your brightness if you have trouble seeing the screen, otherwise it'll be harder to see the targets, especially on the red brick level.
Higher sensitivity makes things better, so go into the options menu and put your sensitivity fairly high (mine is about 75%). You'll be able to look around more easily.
Restart an time you miss a target (if you're going for a perfect score). If you're trying to get the best medal, don't continue playing if you've missed even a single target. Restart the game to save yourself some time.
Unlock more expensive weapons first. Save up your credits and purchase more expensive weapons first. They yield, in some cases, 10 times the amount of lesser weapons. You can then use these to buy cheaper weapons.